I. Field
The present invention relates generally to electronic protection circuitry. More, specifically, the present invention relates to an electrically activated surface mount thermal fuse.
II. Background Details
Protection circuits are often times utilized in electronic circuits to isolate failed circuits from other circuits. For example, the protection circuit may be utilized to prevent a cascade failure of circuit modules in an electronic automotive engine controller. Protection circuits may also be utilized to guard against more serious problems, such as a fire caused by a power supply circuit failure.
One type of protection circuit is a thermal fuse. A thermal fuse functions similar to that of a typical glass fuse. That is, under normal operating conditions the fuse behaves like a short circuit and during a fault condition the fuse behaves like an open circuit. Thermal fuses transition between these two modes of operation when the temperature of the thermal fuse exceeds a specified temperature. To facilitate these modes, thermal fuses include a conduction element, such as a fusible wire, a set of metal contacts, or set of soldered metal contacts, that can switch from a conductive to a non-conductive state. A sensing element may also be incorporated. The physical state of the sensing element changes with respect to the temperature of the sensing element. For example, the sensing element may correspond to a low melting metal alloy or a discrete melting organic compound that melts at an activation temperature. When the sensing element changes state, the conduction element switches from the conductive to the non-conductive state by physically interrupting an electrical conduction path.
In operation, current flows through the fuse element. Once the sensing element reaches the specified temperature, it changes state and the conduction element switches from the conductive to the non-conductive state.
One disadvantage with existing thermal fuses is that during installation of the thermal fuse, care must be taken to prevent the thermal fuse from reaching the temperature at which the sensing element changes state. As a result, existing thermal fuses cannot be mounted to a circuit panel via reflow ovens, which operate at temperatures that will cause the sensing element to open prematurely.